The purpose of this study is to evaluate the safety and effectiveness of the Cementless Oxford Partial Knee System.

The study is designed to document and compare the clinical and radiographic results of the Cementless Oxford Partial Knee System (investigational device) to those of the cemented Oxford Partial Knee System (control device).

Inclusion Criteria:

• have primary diagnosis of osteoarthritis or avascular necrosis limited to the medial compartment of the knee
• signed informed consent for patients contributing to investigational or control if required
• Male or female over the age of 21 years at the time of surgery
• Undergone primary partial knee arthroplasty as unilateral arthroplasty or bilateral arthroplasty, simultaneously or otherwise
• Patients that are current residents of the United Kingdom

Exclusion Criteria:

• Evidence of infection, sepsis, and osteomyelitis at the time of surgery
• Use in the lateral compartment of the knee
• Rheumatoid arthritis or other forms of inflammatory joint disease
• Revision of a failed prosthesis, failed upper tibial osteotomy or post-traumatic arthritis after tibial plateau fracture
• Insufficiency of the collateral, anterior, or posterior cruciate ligaments which would preclude stability of the device
• Partial or full thickness cartilage disease or damage to central part of the lateral femoral condyle
• Osteoporosis or insufficient bone stock
• Metabolic disorders which may impair bone formation
• Osteomalacia
• Distant foci of infection which may spread to the implant site
• Rapid joint destruction, marked bone loss, or bone resorption apparent on roentgenogram
• Vascular insufficiency, muscular atrophy, neuromuscular disease
• Incomplete or deficient soft tissue surrounding the knee
• Charcot's disease
• A fixed varus deformity (not passively correctable) of greater than 15 degrees
• A flexion deformity greater than 15 degrees
• Refuse to sign informed consent if it is required
• Patients that are not current residents of the United Kingdom

A total of 362 cases (121 investigational cases and 241 control cases) for primary efficacy and safety assessment (Primary Comparative Analysis Group); and exploratory assessment from one center in England.

A total of approximately 1,000 investigational cases for "survivorship only" assessment from three centers.

The primary analysis data is patient level data; collected in anticipation of potential PMA submission.